Various photo/optical techniques have been employed to inspect the quality of moving webs. For example, PCT Application WO 84/02190 by J. Kuusi, published June 7, 1984, describes a procedure and means for nondestructively measuring the distribution of filler and/or coating materials in the thickness direction of paper or cardboard. Radiation from a radio-isotope source is employed to excite characteristic X-ray radiation of a component of the material.
U.S. Pat. No. 4,456,374 issued June 26, 1984 to E. Langberg describes a method and apparatus for determining the presence or absence of a coating on a substrate. The technique involves placing the substrate into optical contact with a light guide and utilizing the principle of frustrated total internal reflection. Light scattered from the coating surface is monitored to indicate the presence of a coating.
U.S. Pat. No. 4,490,618 issued Dec. 25, 1984 to P. Cielo describes an apparatus for analyzing the surface of a fibrous web, such as a paper or a textile. The apparatus includes a prism structure, one surface of which is placed in contact with the fibrous web under a predetermined pressure. A collimated light beam is directed into the prism and light reflected from the contact surface through the prism is directed to a detector. The detector senses the light reflected by the contact surface as well as the light diffracted at the contact surface to indicate the surface condition of the fibrous web.
British Patent No. 1,575,140 published Sept. 17, 1980 and issued to Sick GmbH describes an electro-optical monitoring system for checking the presence of creases or other surface irregularities on a moving web of material. The monitoring apparatus directs a sharply defined light beam at a slightly skewed angle relative to a curved surface of the web. A photoelectric light detecting means detects light from the light beam to generate an output signal. The output signal changes in response to the entry of an irregularity into the light beam.
Conventional devices, such as those described above, have not been sufficiently effective for inspecting the registration and location of component elements on a moving web. Mispositioned or missing elements can degrade the quality of certain articles produced from the web. If the inspection system is ineffective, poor quality articles may not be efficiently or accurately culled and removed from the production lot. The alternative procedure of scrapping complete lots based on a limited sample inspection would be excessively wasteful and clearly undesirable. Conventional devices also have not been sufficiently effective for automatically adjusting the production process and machinery to keep all parameters within acceptance specifications. As a result, a production line may have to be stopped creating excessive downtime and reduced production efficiency.